1. Field of the Invention
This invention relates to equipment for welding plastic pipe elements which are melt-weldable, to a welding process in which said equipment is employed, to the pipe fittings designed to be employed with said equipment, as well as to the prefabricated pipe elements obtained by means of said equipment and employing said welding process.
In particular, the invention relates to manifolds, or to a part of the same, or to plants for distribution of water or of other liquid materials, said plants being provided with particular fittings obtained employing the equipment according to the present invention, the manifold-fittings assembly complex being effected with a melt-weldable plastic material, such as for instance polyethylene, polypropylene, etc.
These types of manifolds or branched connections can be employed in any type of plant for the distribution of hot and/or cold liquids, both in an open and in a closed circuit, such as for instance heating or cooling plants, or irrigation plants and so on.
A particular application of the invention is in radiant-panels heating plants, in which plants plastic material pipe coils are provided, as members embedded within concrete castings or within other masonry elements, or within banks (as for instance in agriculture).
2. Related Art
The connection between the manifold fed by the water distribution system and the pipe coil or other plant branches, both made up of a plastic material, or of iron pipes bearing branch pipes of a plastic material, is effected at the present time by means of fittings of various kinds (as for instance pressure fittings, or threaded fittings, or hanger fittings and so on) or by means of combinations thereof.
This type of connection gives rise to a number of different drawbacks, as for instance a loss of seal with time or, sometimes, even an immediate seal loss.
Accordingly, the adaption has been conceived of a welding connection without weld material which is effected by melting the portions to be connected.
In some kinds of plants, as for instance in sewer systems, equipment and welding processes are employed which are based on the operation of heating the surfaces to be joined, by electrically-heated metal surfaces, until incipient melting occurs of the surfaces to be joined. The heated surfaces are then brought in contact with one another by exerting a suitable pressure.
In particular, one of the processes employed at present makes use of a metal plate or "mirror", which has planar, parallel surfaces, and incorporates electrical resistances which heat the plane surfaces up to the desired temperature.
This kind of procedure and the related equipment is advantageously usable for effecting butt welding operations between plane surfaces.
In case it is necessary, as always occurs in the kind of plants mentioned above, to accomplish welding between the pipe or manifold and branches, it is impossible to perform welding operations for branches or parts of pipe coil.
In particular, when the direct welding between the manifold and the pipe coil is adopted, problems occur due to the fact that, while the manifold can undergo length changes because of thermal expansion, the pipe coil can be blocked within the structure of masonry so that it cannot shift transversely. Because of that reason, relevant stresses occur in the connection zone.
Moreover, the welding operation performed at the point corresponding to the intersection between two hollow cylinders having walls of uniform thickness, i.e. between the manifold and the pipe coil, gives a weld whose contact surface consists of a strip of variable width along the circumference of the smallest pipe, from a minimum width equal to the thickness of the pipe itself, up to a higher value, so that non-uniform melting is obtained along the whole contour and as a consequence a poor quality weld is obtained.
In G.B. patent No. 1,246,909, a method and apparatus for welding together tubes of plastics material are described.
The method described comprises machining an approximately semi-cylindrical recess in one of the tubes at the position at which it is to be joined to the other of the tubes to provide it with a joint surface complementary to a joint surface formed by a portion of the wall of the other of the tubes, applying said joint surfaces of the tubes to opposite sides of a heated member shaped to correspond to said joint surfaces to melt the tubes at said joint surfaces, removing the heated member from between said joint surfaces and pressing said joint surfaces of the tubes into abutment one with the other to permit the materials of the tubes to fuse together to form a joint.